Lower urinary tract disorders affect the quality of life of millions of men and women in the United States every year. While the kidneys filter blood and produce urine, the lower urinary tract functions to store and periodically eliminate urine and includes all other parts of the urinary tract except the kidneys. Generally, the lower urinary tract includes the ureters, the urinary bladder, sphincter and the urethra. Disorders of the lower urinary tract include overactive bladder, interstitial cystitis, prostatitis, prostadynia and benign prostatic hyperplasia.
Overactive bladder is a treatable medical condition that is estimated to affect 17 to 20 million people in the United States. Symptoms of overactive bladder can include urinary frequency, urinary urgency, urinary urge incontinence (accidental loss of urine) due to a sudden and unstoppable need to urinate, nocturia (the disturbance of nighttime sleep because of the need to urinate) or enuresis resulting from overactivity of the detrusor muscle (the smooth muscle of the bladder which contracts and causes it to empty).
Neurogenic overactive bladder (or neurogenic bladder) is a type of overactive bladder which occurs as a result of detrusor muscle overactivity referred to as detrusor hyperreflexia, secondary to known neurologic disorders. Patients with neurologic disorders, such as stroke, Parkinson's disease, diabetes, multiple sclerosis, peripheral neuropathy, or spinal cord lesions often suffer from neurogenic overactive bladder. In contrast, non-neurogenic overactive bladder occurs as a result of detrusor muscle overactivity referred to as detrusor muscle instability. Detrusor muscle instability can arise from non-neurological abnormalities, such as bladder stones, muscle disease, urinary tract infection or drug side effects or can be idiopathic.
Due to the enormous complexity of micturition (the act of urination) an exact mechanism which causes overactive bladder is not known. Overactive bladder can result from hypersensitivity of sensory neurons of the urinary bladder, arising from various factors including inflammatory conditions, hormonal imbalances, and prostate hypertrophy. Destruction of the sensory nerve fibers, either from a crushing injury to the sacral region of the spinal cord, or from a disease that causes damage to the dorsal root fibers as they enter the spinal cord can also lead to overactive bladder. In addition, damage to the spinal cord or brain stem causing interruption of transmitted signals can lead to abnormalities in micturition. Therefore, both peripheral and central mechanisms can be involved in mediating the altered activity in overactive bladder.
In spite of the uncertainty regarding whether central or peripheral mechanisms, or both, are involved in overactive bladder, many proposed mechanisms implicate neurons and pathways that mediate non-painful visceral sensation. Somatosensory information from the bladder is relayed by nociceptive Aδ and C fibers that enter the spinal cord via the dorsal root ganglion (DRG) and project to the brainstem and thalamus via second or third order neurons (Andersson (2002) Urology 59:18–24; Andersson (2002) Urology 59:43–50; Morrison, J., Steers, W. D., Brading, A., Blok, B., Fry, C., de Groat, W. C., Kakizaki, H., Levin, R., and Thor, K. B., “Basic Urological Sciences” In: Incontinence (vol. 2) Abrams, P. Khoury, S., and Wein, A. (Eds.) Health Publications, Ltd., Plymbridge Ditributors, Ltd., Plymouth, UK., (2002). A number of different subtypes of sensory afferent neurons can be involved in neurotransmission from the lower urinary tract. These can be classified as, but not limited to, small diameter, medium diameter, large diameter, myelinated, unmyelinated, sacral, lumbar, peptidergic, non-peptidergic, IB4 positive, IB4 negative, C fiber, Aδ fiber, high threshold or low threshold neurons. Nociceptive input to the DRG is thought to be conveyed to the brain along several ascending pathways, including the spinothalamic, spinoreticular, spinomesencephalic, spinocervical, and in some cases dorsal column/medial lemniscal tracts (A. I. Basbaum and T. M. Jessell (2000) The perception of pain. In Principles of Neural Science, 4th. ed.).
Currently there are no clinically approved applications of central nervous system oriented pharmacotherapies for treating lower urinary tract disorders, such as overactive bladder. However, recent animal studies have suggested potential targets in the central nervous system for modulating urinary tract functions. For example, in the raphe nucleus of the caudal brain stem, 5-hydroxytryptamine (serotonin, 5-HT) containing neurons send projections to the dorsal horn as well as to the autonomic and sphincter motor nuclei in the lumbosacral spinal cord. The sympathetic and parasympathetic autonomic nuclei as well as the sphincter motor nuclei receive prominent serotonergic input from the raphe nuclei in the caudal brain stem. Activity in the serotonergic pathway generally enhances urine storage by facilitating the vesical sympathetic reflex pathway and inhibiting the parasympathetic voiding pathway (Sharma, A. et al. (2000) Pharmacokinetics and safety of duloxetine, a dual-serotonin and norepinephrine reuptake inhibitor. J. Clin. Pharmacol. 40: 161 and Thor, K. B. et al. (1995) Effects of duloxetine, a combined serotonin and norephinephrine reuptake ihibitor, on central neural control of lower urinary tract function in the chloralose-anesthetized female cat. J. Pharmacol. Exp. Ther. 274: 1016.)
Among the various subtypes of 5-HT receptors, 5-HT2 and 5-HT3 receptors mediate excitatory effects on sympathetic and somatic reflexes to increase outlet resistance. Moreover, 5-HT2C and 5-HT3 receptors have also been shown to be involved in inhibition of the micturition reflex (Downie, J. W. (1999) Pharmacological manipulation of central micturition circuitry. Curr. Opin. SPNS Inves. Drugs 1:23). In fact, 5-HT3 receptor inhibition has been shown to diminish 5-HT mediated contractions in rabbit detrusor (Khan, M. A. et al. (2000) Doxazosin modifies serotonin-mediated rabbit urinary bladder contraction. Potential clinical relevance. Urol. Res. 28:116).
Current treatments for overactive bladder include medication, diet modification, programs in bladder training, electrical stimulation, and surgery. Currently, antimuscarinics (which are members of the general class of anticholinergics) are the primary medication used for the treatment of overactive bladder. The antimuscarinic, oxbutynin, has been the mainstay of treatment for overactive bladder. However, treatment with antimuscarinics suffers from limited efficacy and side effects such as dry mouth, dry eyes, dry vagina, blurred vision, cardiac side effects, such as palpitations and arrhythmia, drowsiness, urinary retention, weight gain, hypertension and constipation, which have proven difficult for some individuals to tolerate.
Interstitial cystitis is another lower urinary tract disorder of unknown etiology that predominantly affects young and middle-aged females, although men and children can also be affected. Symptoms of interstitial cystitis can include irritative voiding symptoms, urinary frequency, urinary urgency, nocturia or suprapubic or pelvic pain related to and relieved by voiding. Many interstitial cystitis patients also experience headaches as well as gastrointestinal and skin problems. In some cases, interstitial cystitis can also be associated with ulcers or scars of the bladder.(Metts, J. F. (2001) Interstitial Cystitis: Urgency and Frequency Syndrome. American Family Physician 64(7): 1199–1206).
Currently, the only FDA-approved oral medication for use in interstitial cystitis is ELMIRON® (pentosan polysulfate sodium). ELMIRON® was approved in 1996 and is thought to work by restoring a damaged, thin or leaky bladder surface. However, ELMIRON® must be taken continually for several months before any improvements can be expected. As such, lack of patient compliance often results in unsuccessful treatment. In addition, treatment with ELMIRON® is not effective in a large percentage of patients.
Other medications which have been used “off-label” for the treatment of interstitial cystitis include, for example, antidepressants, antihistamines and anticonvulsants (See, Theoharides, T. C. (2001) New agents for the medical treatment of interstitial cystitis. Exp. Opin. Invest. Drugs 10(3): 521–46). However, in view of the unknown cause of interstitial cystitis and the suggestion that the disorder is multifactorial in origin, these additional therapies have not provided adequate relief of the associated symptoms.
Prostatitis and prostadynia are other lower urinary tract disorders that have been suggested to affect approximately 2–9% of the adult male population (Collins M. M. et al., (1998) “How common is prostatitis? A national survey of physician visits,” Journal of Urology, 159: 1224–1228). Prostatitis is an inflammation of the prostate, and includes bacterial prostatitis (acute and chronic) and non-bacterial prostatitis. Acute and chronic bacterial prostatitis are characterized by inflammation of the prostate and bacterial infection of the prostate gland, usually associated with symptoms of pain, urinary frequency and/or urinary urgency. Chronic bacterial prostatitis is distinguished from acute bacterial prostatitis based on the recurrent nature of the disorder. Chronic non-bacterial prostatitis is characterized by inflammation of the prostate which is of unknown etiology accompanied by the presence of an excessive amount of inflammatory cells in prostatic secretions not currently associated with bacterial infection of the prostate gland, and usually associated with symptoms of pain, urinary frequency and/or urinary urgency. Prostadynia is a disorder which mimics the symptoms of prostatitis absent inflammation of the prostate, bacterial infection of the prostate and elevated levels inflammatory cells in prostatic secretions. Prostadynia can be associated with symptoms of pain, urinary frequency and/or urinary urgency.
Currently, there are no established treatments for prostatitis and prostadynia. Antibiotics are often prescribed, but with little evidence of efficacy. COX-2 selective inhibitors and α-adrenergic blockers and have been suggested as treatments, but their efficacy has not been established. Hot sitz baths and anticholinergic drugs have also been employed to provide some symptomatic relief.
Benign prostatic hyperplasia (BPH) is a non-malignant enlargement of the prostate that is very common in men over 40 years of age. BPH is thought to be due to excessive cellular growth of both glandular and stromal elements of the prostate. Symptoms of BPH can include urinary frequency, urinary urgency, urge incontinence, nocturia, or reduced urinary force and speed of flow.
Invasive treatments for BPH include transurethral resection of the prostate, transurethral incision of the prostate, balloon dilation of the prostate, prostatic stents, microwave therapy, laser prostatectomy, transrectal high-intensity focused ultrasound therapy and transurethral needle ablation of the prostate. However, complications can arise through the use of some of these treatments, including retrograde ejaculation, impotence, postoperative urinary tract infection and some urinary incontinence. Non-invasive treatments for BPH include androgen deprivation therapy and the use of 5α-reductase inhibitors and α-adrenergic blockers. However, these treatments have proven only minimally to moderately effective for some patients.
In view of the limitations associated with existing therapies and treatments for lower urinary tract disorders, new therapies and treatments are highly desirable.